The present invention relates to an apparatus for measuring the speed of an object using the ultrasonic sound wave, and particularly to an apparatus for measuring the blood flow speed in a biological body on a real-time basis.
Among the flow speed measuring apparatus based on Doppler effect of the ultrasonic sound wave, one system uses the pulsed continuous wave. The system generally has a highest measurable Doppler frequency 1/2T for the pulse transmission repetition or iteration period T. Accordingly, by choosing the value of T in compliance with the distance between the probe for transmission and reception of the ultrasonic wave and the target object, the highest measurable flow speed is determined.
The inventors of the present invention proposed in U.S. Pat. No. 4,751,847 a new pulse Doppler flow speed meter which sets two transmission iteration periods T and (T+Ts) so that the highest measurable Doppler frequency is 1/2Ts, and consequently the above-mentioned highest measurable flow speed is increased by a factor of T/Ts. This improved pulse Doppler flow speed meter, however, has an inferior S/N ratio if the reflected sound wave from the flowing object is weaker than that from stationary objects, resulting possibly in the inaccurate flow speed measurement.